Derek, you will end up with a decent brisket roast if you do it the way your butcher has suggested. It will not be great though. Although I also suggest brisket method suggested by Fweck, here is a different point of view.
1. Slow cooking a brisket in an electric oven (you did not say what kind of oven) will result in a very dry environment that the brisket cooks in. The risk is that the meat will develop a dry surface, brisket can get a splintery texture this way. In a gas oven, it will not develop the proper bark.
2. In most processes, brisket is cooked at least part of the time in a moist environment, this includes smoking in fact. You typical oven vents in a manner that this is not achievable without foiling or braising. This is why so many non-bbq processes add moisture in the cooking process. I suggest whatever method you use, consider using a water pan, foiling or other means for increasing the moisture in the oven.
3. I do not recommend using dry rubs in an indoor oven, as they can cause smoking, not good smoking, indoors. Never good.
4. If the purpose of this brisket cook is to produce a good brisket for dinner, proceed as your butcher described. If you want to learn something about what most of us consider to be good brisket, you might at least take a careful look at that Barbefunkoramaque link and get an understanding of one way people have learned to cook.
5. If you want another option, simplify, rub with garlic, kosher salt, medium grained pepper and Mrs, Dash Lemon Pepper, then toss in the oven, oven pan filled half full of water on the bottom, brisket on a rack above, and let her rip at 325F for 4 to 5 hours. Probe at 4 hours and keep checking every 30 minutes until a thermometer probe goes in real easy.
